In hindsight, letitrip's first piece of advice is the best in my experience.
I was asked to do this once, before I actually got into doing karaoke. I was in the house band at a nearby music pub. I thought yeah, this should be fun. WRONG!!!
We did a voting system, so nobody knew who voted for who, but at the end of the show, there was a girl who had brought all her family & friends and they were all in our faces going nuts because she didn't win.
Thankfully, there was no violence, but it got very close. They were all hanging around outside the pub for ages after they walked out. I quietly stepped through the side door and tippy toe'd it to my car.
Never again!
The only system I have ever used since is to put a stamp on the slips and put them in a bucket. Only those slips handed to me until 10pm went into the bucket. This encouraged people to arrive earlier. Not sure if that will work in today's recession, but it worked just great at the time. At the end of the night, usually about half an hour before closing, we simply picked a random slip out of the hat and that person won £50. Of couse, they'd go & buy all their mates a drink so the pub owners are happier still.
The singers were happy because all they needed to do was sing so everybody had a chance and the pub was happy because it got the punters in early and kept them there.
